The present invention relates to a process for producing lithium-cobalt oxide represented by the formula: Li.sub.x CoO.sub.2 (0&lt;x&lt;1), and more particularly, to a process for producing lithium-cobalt oxide particles represented by the formula: Li.sub.x CoO.sub.2 (0&lt;x&lt;1), useful especially as a cathode active substance for lithium ion batteries, which can be produced by calcination in a short time, and has a narrow particle size distribution and a uniform small particle size.
In recent years, in association with developments of personal computers and portable equipment such as portable telephones, there have been an increasing demand for batteries as a power source therefor. Especially, a lot of earnest studies for lithium ion batteries have been made in various fields, because lithium is expected to provide batteries having a high electromotive force and a high energy density due to a small atomic weight and a high ionization energy thereof.
In addition, as a cathode active substance usable in lithium ion batteries, lithium-cobalt oxide (LiCoO.sub.2), lithium-nickel oxide (LiNiO.sub.2) or the like have been earnestly studied because these substances can generate a voltage as high as 4 V. These compounds such as lithium-cobalt oxide or lithium-nickel oxide have been conventionally produced by mixing cobalt- or nickel-containing oxides as a raw material with a lithium compound and calcining the obtained mixture at a temperature as high as not less than 500.degree. C. (U.S. Pat. No. 4,302,518).
However, in the case of such high-temperature calcination method, it is necessary to calcine the obtained mixture for a long period of time, because the cobalt oxide particles have a low reactivity upon solid phase reaction thereof. When the mixture are calcined at such a high temperature for a long period of time, lithium contained in the lithium compound is evaporated. For this reason, the loss of lithium is caused and the composition of the obtained lithium-cobalt oxide tends to be fluctuated, resulting in such a disadvantage that it is difficult to produce lithium-cobalt oxide having a stable quality.
In addition, when the lithium-cobalt oxide particles or the lithium-nickel oxide particles are used as a cathode active substance for lithium ion batteries, these particles are first dispersed in a binder to prepare a coating material, and then the coating material is applied onto a metal plate such as copper and dried to form a cathode active substance for the lithium ion batteries. However, since the particles are calcined at a high temperature for a long period of time, the resultant lithium-cobalt oxide particles are firmly fused together. Because of the production of the coating material for cathode, such fused lithium-cobalt oxide particles must be intensely pulverized into an appropriate particle size. This causes the increase in cost of energy required. Also, there arises a problem that a grinding body used for the pulverization is abraded and disadvantageously mixed in the obtained lithium-cobalt particles.
Further, when used as a cathode active substance, since the lithium-cobalt oxide particles are dispersed in a binder and the resultant coating material is applied onto a metal plate such as a copper plate and dried to form a cathode for the lithium ion batteries, it is important that the lithium-cobalt oxide particles have a uniform particle shape and a uniform particle size, because the higher the packing density of particles in the coating layer, the higher the capacity of battery obtained becomes.
Under these circumstances, a process for the production of lithium-cobalt oxide as a cathode active substance for the lithium ion batteries, which have a narrow particle size distribution and a uniform particle size, which process comprises calcining raw particles in a short time, has been demanded.
As a result of the present inventors' earnest studies, it has been found that by mixing specific cobalt oxide with a specific lithium compound and calcining the obtained mixture at a specific temperature, the obtained lithium-cobalt oxide particles have a narrow particle size distribution and a uniform particle size. The present invention has been attained on the basis of this finding.